Refrigeration systems, such as those used to refrigerate cold rooms, are susceptible to frost build up on e.g. the evaporator coils of such systems (although frost is not limited to this part of the system).
One way to remove this frost is to use a hot gas defrost system. In existing hot gas defrost systems, refrigerant vapour from the compressor discharge or high pressure receiver of the refrigeration system is diverted through the evaporator coils. The hot vapour condenses in the evaporator coils, thereby releasing heat and causing the frost to melt. This type of system can result in large energy losses due to more condensing energy being released than is needed for frost removal, and requires a large reservoir for accommodating the condensate formed during defrost. This type of system also conveys refrigeration machine oil contained in the compressor discharge to the evaporators being defrosted. This is undesirable because it can result in oil fouling within the evaporators that, in turn, results in a reduction in efficiency of the coils and the system. In cases where the refrigerant is toxic, flammable or environmentally harmful, it is desirable to minimise the system's refrigerant inventory. For example, accidents associated with known hot gas defrost systems have been caused by phenomena such as liquid hammer, hydraulic shock and subsequent pipe rupture.
Defrost systems that do not necessarily increase the system's refrigerant inventory are known. These are, for example, electric defrost, ambient air defrost and water defrost. Electric defrost uses high grade energy for a simple heating purpose. This type of defrost is often highly inefficient and unreliable (requiring frequent heater replacement). On the other hand, ambient air defrost can be efficient, but is dependent on climatic conditions and system design (and is thus not always possible). Similarly, water defrost can be efficient but can also malfunction, causing water damage in the refrigerated space (e.g. warehouse) as a result of drain pan overflows not being recognised in time.
A further type of defrost system is a warm glycol defrost system. This type of system consists of a glycol tank that is warmed by a discharge gas (of the refrigeration system). When the defrost system is activated, warm glycol is pumped (by a separate circulation pump) through the evaporator and drip tray of the refrigeration system in glycol tubes that are separate to the refrigeration system. In general, the film coefficient between the glycol and internal surfaces of the glycol tubes at the evaporator is relatively low. This is compensated for by way of elevated glycol inlet temperatures. However, the requirement for elevated inlet temperatures precludes the use of low grade heat sources (e.g. having temperatures of 5° C. to 10° C. above the freezing point of water) in these types of defrost systems.
The above references to the background art do not constitute an admission that the art forms part of the common general knowledge of a person of ordinary skill in the art. The above references are also not intended to limit the application of the defrost system, refrigeration system and method as disclosed herein.